This is a case of a 33-year-old G2 P1 was referred for the ultrasound scan at 28 weeks of gestation. The ultrasound examination revealed a large cyst, 40 mm in diameter localized anterior to the cerebellum, at the level of the interpeduncular cistern.
Our differential diagnosis included porencephalic cyst, arachnoid cyst, glioependymal cyst.

The fetal MRI performed afterwards confirmed the ultrasound findings. The MRI report was following: arachnoidal cyst at the basal (interpeduncular) cistern with the continuation to the 3rd ventricle, temporal horn of right lateral ventricle and prepontine cistern causing the right temporal lobe atrophy. The frontal lobes and corpus callosum were intact.

Patient was scheduled for the cesarean section followed by the surgery with the shunt insertion.
Images 1,2: 28 weeks; axial view of the fetal brain, no dilatation of the posterior horns of the lateral ventricles.

Images 5-9: Fetal MRI, 32 weeks; images show the large cyst (40 x 42 mm) located at interpeduncular (basal) cistern with continuation to the 3rd ventricle, temporal horn of the right ventricle and prepontine cistern.